Archive for the ‘Hot Articles’ Category

Geographer Vaclav Smil described the Haber-Bosch process as the “detonator of the human population explosion” in the twentieth century, in his Nature Millennium Essay.1 Today, nearly 80% of nitrogen atoms in human tissue have been through the Haber-Bosch process;2 where nitrogen gas is converted to ammonia converted into industrial fertilizers.

The Haber-Bosch process has now entered its second century. High temperatures and pressures and a catalyst composed of magnetite (Fe3O4), wüstite (FeO) and iron(0) metal, push the equilibrium of a mixture of pure hydrogen, nitrogen and ammonia gas towards the formation of ammonia. Today, one of the greatest challenges of industrial chemistry is to find an alternative catalyst and process.

In 1991 Leigh et. al. reported the nitrogen of nitrogen by a homogeneous Fe complex with two chelating phosphine ligands.3 They were able to reduce N2 to ammonia (isolated as NH4+) under strongly acidic conditions. However, following this discovery, verification and mechanistic questions remained.

The previously unreported dimer

In a recent article, ‘Teaching old compounds new tricks: efficient N2 fixation by simple Fe(N2)(diphosphine)2 complexes‘ published in Dalton Transactions, , Ashley and co-workers report their investigation of the Leigh compound. They have persued a peak that was previously unaccounted for in the 31P NMR spectrum which has led them to isolate a unique dimer of this complex, bridged by molecular N2. Comparing the reactivities of this dimer with the two monomers that feature different simple chelating phosphine ligands, they unambiguously report yields of NH3 and N2H4 after reaction with triflic acid, and discern dependences based on ligand, temperature, and solvent.

This hitherto unreported dimeric compound, and the impressive NH3/N2H4 yields achieved with the monomers tested, add a significant piece to the puzzle of how iron-mediated N2 activation occurs.

Take a look at our HOT articles for 2016 which are free to access for 4 weeks and will be updated regularly so keep checking! These have also been compiled into a collection for viewing on our website.

“My original intention in the late 1940s was to spend a few years understanding the boranes, and then to discover a systematic valence description of the vast numbers of electron deficient intermetallic compounds. I have made little progress toward this latter objective,” said the late Professor William N. Lipscomb in his 1976 Nobel acceptance speech.1

In their recent Dalton Transactions Hot Article, Jose M. Goicoechea and John E. McGrady examine the chemistry of main group cluster-encapsulated transition metal atoms, laying another piece of the foundation of Lipscomb’s “latter objective.”

The authors assign themselves an ambitious task: to provide a system to predict the geometries of cages of the tetrel elements (C, Si, Ge, Sn, Pb) which encapsulate transition metal atoms. They focus on six high-symmetry cage structures, shown below, which have been observed for tetrel-encapsulated metal atoms (denoted M@Ex, for example Ni@Ge12.).

The six three-dimensional cage geometries examined.

Lipscomb’s elegantly-described closo, nido, and arachno borane structures (“closed,”“nest”, and “spider’s web,” respectively) provided an initial basis for classifications of cages. Later, the Wade/Mingos rules laid the foundation for predicting the geometries based on the electronic structure of the cluster.

Goicoechea and McGrady use the total valence electron count – of the tetrel cages, plus the d-electron count of the encapsulated metal – to describe patterns in the structures. Nevertheless, some results defy electron-count classification, for example, the preference of silicon cages to form D6h-symmetric hexagonal prisms in M@Si12 complexes, in contrast to the M@Ge12 analogues.

It is a broad, big-picture paper, a synthesis of a wide range of experimental and theoretical results. Some structures are known experimentally from x-ray crystallography, some have only been predicted computationally. The authors discuss the varying relevance of considering the d-electron counts of the metals, and technological implications such as quenching of the magnetic moment of encapsulated metal atoms. For me, the scope alone made this a worthwhile read.

Ian Mallov is currently a Ph.D. student in Professor Doug Stephan’s group at the University of Toronto. His research is focused on synthesizing new Lewis-acidic compounds active in Frustrated Lewis Pair chemistry. He grew up in Truro, Nova Scotia and graduated from Dalhousie University and the University of Ottawa, and worked in chemical analysis in industry for three years before returning to grad school.